Duo-servo drum brake

Abstract

Counter braking force acting on a secondary shoe is input to one end portion of a control lever supported with an anchor pin vertically provided on a backing plate in such a manner that the control lever is capable of swinging. An inner end portion in the radial direction of the control lever engages with a strut for pressing a primary shoe against a brake drum. Pressing force A with which the strut is pressing the primary shoe against the brake drum is drastically reduced when braking force, that is, the counter braking force increases and slightly reduced when the braking force, that is, the counter braking force decreases, whereby an increase or a decrease in the braking force is made automatically controllable.

What is claimed is: 
     
       1. A duo-servo drum brake comprising: a primary shoe and a secondary shoe which are disposed opposite to each other inside a brake drum, counter braking force acting on the primary shoe being used for pressing the secondary shoe against the brake drum; and an anchor pin disposed forward further than the secondary shoe in the direction in which the brake drum is rotated forward and used for receiving the counter braking force, further including: a shoe expanding mechanism having a first link for pressing the primary shoe against the brake drum and a second link for pressing the secondary shoe synchronously when the first link presses the primary shoe;   a control lever which is held around the anchor pin and when its end portion on one side receives counter braking force acting on the secondary shoe, its end portion on the other side is capable of swinging around the anchor pin in the direction of the secondary shoe;   a balance link which is pivotally supported with the secondary shoe in a manner capable of swinging, has a contact surface in one end portion with the other end portion of the control lever and a contact surface in the other end portion with a pressing portion of the second link, and swings when the pressing force received with the contact surface in the one end portion from the control lever exceeds the pressing force received with the contact surface in the other end portion from the second link; and   a projected portion which is provided for the first link and when the other end portion of the control lever is displaced toward the secondary shoe after swinging the balance link, engages with the secondary shoe in order to reduce the pressing force of the first link.